Strings in Python

String literals are indicated by double quotes "a". Unlike some other languages, Python is not too picky about single or double quotes to indicate strings, but double quotes are usually preferred for multi-character strings. If a string contains an apostrophe or its own quotes, the surrounding quotes must be of the other type.

s1="This is a string."
s2="It's time to go."
s3='The man said, "Time to go."'

The length of a string can be dynamically determined when the script is run, but once set, it is fixed because strings are immutable. The string variable can be overwritten, however.

Line_1="The first line of a file\n"

The \n symbol represents a new line and is treated as a single character. The length of the above string is 25; spaces and the newline count.

If a string literal is surrounded by triple double quotes """s""" it is verbatim, including newlines typed.

s="""This string is a
     multiline quote."""

If evaluated as an expression, the string will show the newline. If the print function is used, it will print exactly as typed.

String Comparisons

String comparisons use the same familiar symbols as arithmetic comparisons, but with lexical ordering. This can result in some surprises if the strings represent numbers. Never forget that strings are a completely different type from the numbers they may seem to represent! Equality also requires exact equality, including spaces, matching cases, etc.

• Equality
  • ==
• Lexically greater than or lexically greater than or equal
  • \> \>=
• Lexically less than or lexically less than or equal
  • < <=

Example

s1="This is a string."
s2="That is a string."
s3="This is a string"  #no period
print(s1==s3)
print(s1<=s2)

Exercise

number_1="10"
number_2="2"
print(number_1 < number_2)

String Operators and Functions

Python supplies many string operators and functions. Among the most commonly used are

• Concatenation
  • s1 + s2
• Number of characters
  • len(string)
• Type conversion from numerical type to string
  • str(f)
• Type conversion from string to numerical type. This must be possible according to the interpreter’s rules for the numbers. In particular, the string "3." does not represent an integer.
  • float(s)
• Raw string: no characters are taken to be special characters. Sometimes particularly useful on Windows. Either r or R can be used.
  • r'This is a string \ with no special characters \n'

Exercise

Type in the following code. What causes the difference?

s1="Today \n is a new day."
s2=r"Today \n is a new day."
print(s1)
print(s2)

Define variables x=21.0, n=30, s="My new string." Convert n into a float and store the results into a new variable y. Set a variable the_answer containing the literal string “42.” (be sure to include the period). Type

z=int(the_answer)

What happened? Try

z=float(the_answer)

Substrings

Although a particular string variable is immutable, it is possible to extract substrings from it.

sub_string=string[0:3]

In this context the colon (:) is again representing the range operator. Recall that for all ordered types, Python counts from zero. So the first character is numbered 0, the second is 1, and so forth. As we have seen before, the upper bound is always exclusive in Python. Thus the variable sub_string consists of characters 0, 1, and 2.

Since strings are immutable we cannot assign values to a substring; that is, they cannot appear on the left-hand sign of an assignment = statement.

Exercise

Type into the Spyder interpreter pane or a JupyterLab notebook. Remember that in Jupyter each evaluation expression should be run in its own cell.

title="This is a string."
subtitle="This is another string."
len(title)
title+":"+subtitle
newtitle=title+" : "+subtitle
len(newtitle)
newtitle[2:4]="at"  #Error-why?
x=19.58
print("The value of x is {:f}".format(x))